diff options
| -rw-r--r-- | Documentation/lockup-watchdogs.txt | 63 | ||||
| -rw-r--r-- | Documentation/nmi_watchdog.txt | 83 |
2 files changed, 63 insertions, 83 deletions
diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt new file mode 100644 index 00000000000..d2a36602ca8 --- /dev/null +++ b/Documentation/lockup-watchdogs.txt @@ -0,0 +1,63 @@ +=============================================================== +Softlockup detector and hardlockup detector (aka nmi_watchdog) +=============================================================== + +The Linux kernel can act as a watchdog to detect both soft and hard +lockups. + +A 'softlockup' is defined as a bug that causes the kernel to loop in +kernel mode for more than 20 seconds (see "Implementation" below for +details), without giving other tasks a chance to run. The current +stack trace is displayed upon detection and, by default, the system +will stay locked up. Alternatively, the kernel can be configured to +panic; a sysctl, "kernel.softlockup_panic", a kernel parameter, +"softlockup_panic" (see "Documentation/kernel-parameters.txt" for +details), and a compile option, "BOOTPARAM_HARDLOCKUP_PANIC", are +provided for this. + +A 'hardlockup' is defined as a bug that causes the CPU to loop in +kernel mode for more than 10 seconds (see "Implementation" below for +details), without letting other interrupts have a chance to run. +Similarly to the softlockup case, the current stack trace is displayed +upon detection and the system will stay locked up unless the default +behavior is changed, which can be done through a compile time knob, +"BOOTPARAM_HARDLOCKUP_PANIC", and a kernel parameter, "nmi_watchdog" +(see "Documentation/kernel-parameters.txt" for details). + +The panic option can be used in combination with panic_timeout (this +timeout is set through the confusingly named "kernel.panic" sysctl), +to cause the system to reboot automatically after a specified amount +of time. + +=== Implementation === + +The soft and hard lockup detectors are built on top of the hrtimer and +perf subsystems, respectively. A direct consequence of this is that, +in principle, they should work in any architecture where these +subsystems are present. + +A periodic hrtimer runs to generate interrupts and kick the watchdog +task. An NMI perf event is generated every "watchdog_thresh" +(compile-time initialized to 10 and configurable through sysctl of the +same name) seconds to check for hardlockups. If any CPU in the system +does not receive any hrtimer interrupt during that time the +'hardlockup detector' (the handler for the NMI perf event) will +generate a kernel warning or call panic, depending on the +configuration. + +The watchdog task is a high priority kernel thread that updates a +timestamp every time it is scheduled. If that timestamp is not updated +for 2*watchdog_thresh seconds (the softlockup threshold) the +'softlockup detector' (coded inside the hrtimer callback function) +will dump useful debug information to the system log, after which it +will call panic if it was instructed to do so or resume execution of +other kernel code. + +The period of the hrtimer is 2*watchdog_thresh/5, which means it has +two or three chances to generate an interrupt before the hardlockup +detector kicks in. + +As explained above, a kernel knob is provided that allows +administrators to configure the period of the hrtimer and the perf +event. The right value for a particular environment is a trade-off +between fast response to lockups and detection overhead. diff --git a/Documentation/nmi_watchdog.txt b/Documentation/nmi_watchdog.txt deleted file mode 100644 index bf9f80a9828..00000000000 --- a/Documentation/nmi_watchdog.txt +++ /dev/null @@ -1,83 +0,0 @@ - -[NMI watchdog is available for x86 and x86-64 architectures] - -Is your system locking up unpredictably? No keyboard activity, just -a frustrating complete hard lockup? Do you want to help us debugging -such lockups? If all yes then this document is definitely for you. - -On many x86/x86-64 type hardware there is a feature that enables -us to generate 'watchdog NMI interrupts'. (NMI: Non Maskable Interrupt -which get executed even if the system is otherwise locked up hard). -This can be used to debug hard kernel lockups. By executing periodic -NMI interrupts, the kernel can monitor whether any CPU has locked up, -and print out debugging messages if so. - -In order to use the NMI watchdog, you need to have APIC support in your -kernel. For SMP kernels, APIC support gets compiled in automatically. For -UP, enable either CONFIG_X86_UP_APIC (Processor type and features -> Local -APIC support on uniprocessors) or CONFIG_X86_UP_IOAPIC (Processor type and -features -> IO-APIC support on uniprocessors) in your kernel config. -CONFIG_X86_UP_APIC is for uniprocessor machines without an IO-APIC. -CONFIG_X86_UP_IOAPIC is for uniprocessor with an IO-APIC. [Note: certain -kernel debugging options, such as Kernel Stack Meter or Kernel Tracer, -may implicitly disable the NMI watchdog.] - -For x86-64, the needed APIC is always compiled in. - -Using local APIC (nmi_watchdog=2) needs the first performance register, so -you can't use it for other purposes (such as high precision performance -profiling.) However, at least oprofile and the perfctr driver disable the -local APIC NMI watchdog automatically. - -To actually enable the NMI watchdog, use the 'nmi_watchdog=N' boot -parameter. Eg. the relevant lilo.conf entry: - - append="nmi_watchdog=1" - -For SMP machines and UP machines with an IO-APIC use nmi_watchdog=1. -For UP machines without an IO-APIC use nmi_watchdog=2, this only works -for some processor types. If in doubt, boot with nmi_watchdog=1 and -check the NMI count in /proc/interrupts; if the count is zero then -reboot with nmi_watchdog=2 and check the NMI count. If it is still -zero then log a problem, you probably have a processor that needs to be -added to the nmi code. - -A 'lockup' is the following scenario: if any CPU in the system does not -execute the period local timer interrupt for more than 5 seconds, then -the NMI handler generates an oops and kills the process. This -'controlled crash' (and the resulting kernel messages) can be used to -debug the lockup. Thus whenever the lockup happens, wait 5 seconds and -the oops will show up automatically. If the kernel produces no messages -then the system has crashed so hard (eg. hardware-wise) that either it -cannot even accept NMI interrupts, or the crash has made the kernel -unable to print messages. - -Be aware that when using local APIC, the frequency of NMI interrupts -it generates, depends on the system load. The local APIC NMI watchdog, -lacking a better source, uses the "cycles unhalted" event. As you may -guess it doesn't tick when the CPU is in the halted state (which happens -when the system is idle), but if your system locks up on anything but the -"hlt" processor instruction, the watchdog will trigger very soon as the -"cycles unhalted" event will happen every clock tick. If it locks up on -"hlt", then you are out of luck -- the event will not happen at all and the -watchdog won't trigger. This is a shortcoming of the local APIC watchdog --- unfortunately there is no "clock ticks" event that would work all the -time. The I/O APIC watchdog is driven externally and has no such shortcoming. -But its NMI frequency is much higher, resulting in a more significant hit -to the overall system performance. - -On x86 nmi_watchdog is disabled by default so you have to enable it with -a boot time parameter. - -It's possible to disable the NMI watchdog in run-time by writing "0" to -/proc/sys/kernel/nmi_watchdog. Writing "1" to the same file will re-enable -the NMI watchdog. Notice that you still need to use "nmi_watchdog=" parameter -at boot time. - -NOTE: In kernels prior to 2.4.2-ac18 the NMI-oopser is enabled unconditionally -on x86 SMP boxes. - -[ feel free to send bug reports, suggestions and patches to - Ingo Molnar <mingo@redhat.com> or the Linux SMP mailing - list at <linux-smp@vger.kernel.org> ] - |